1. Field of the Invention
The present invention relates to printed substrate, and, more particularly to a printed substrate having reinforcement patterns of different size and/or shape formed around through-holes in different layers of the printed substrate to distribute stress on the borders of the reinforcement patterns.
2. Description of the Related Art
Printed substrates, such as flexible printed substrates, having devices secured thereon are known. When devices are mounted on the flexible printed substrate, the relative positional relationship between the mounted devices and the flexible printed substrate is maintained by securing the devices with pins which pass through holes provided on the flexible printed substrate (hereinafter referred to as "through-holes"). Further, when inspecting a flexible printed substrate on which devices have been mounted, the flexible printed substrate is secured by passing pins through the through-holes provided in the flexible printed substrate to secure the relative position of electric terminal seats when inspecting for signal exposure using an inspection device probe. When securing the flexible printed substrate, the through-holes of the flexible printed substrate may be broken during the operations of installation or removal of the pins which secure the devices or the flexible printed substrate because there is a difference in the rigidity of the pins, which are made of metal, and the rigidity of the base material of the flexible printed substrate, which is made of a synthetic resin.
FIGS. 5A, 5B and 6 show a conventional flexible printed substrate 21 having a structure which prevents breakage of the flexible printed substrate 21. FIG. 5A is a view of a front surface of the flexible printed substrate 21. Circular reinforcement patterns 24 and 25 are formed around the perimeters of through-holes 22 and 23 and are concentric with through-holes 22 and 23, respectively. The reinforcement patterns 24, 25 are made of copper having a rigid strength which is close to the rigid strength of the pins (not shown) which are placed through the through-holes. FIG. 5B is a view of a back surface of flexible printed substrate 21 showing circular reinforcement patterns 26 and 27 formed around the perimeters of through-holes 22 and 23 and concentric with through-holes 22 and 23, respectively.
FIG. 6 is a partial cross-sectional diagram of the conventional flexible printed substrate 21 showing a region near through-hole 22. Circular reinforcement pattern 24 is formed on the front surface of the flexible printed substrate 21, and circular reinforcement pattern 26 is formed on the back surface of the substrate 21. Further, reinforcement patterns 24 and 26 have the same shape. Moreover, reinforcement patterns 24 and 26 and flexible printed substrate 21 are covered by protective cover films 28 and 29.
When providing circular reinforcement patterns 24 and 26 around the perimeters of through-holes 22 and 23, respectively, of the flexible printed substrate 21, the rigidity of the flexible printed substrate is raised to a great extent by making reinforcement patterns 24 and 26 the same shape on the front and back of the substrate 21. Instead of preventing breakage at the contact terminal surface of the pins and through-holes 22, 23, stress accumulates at a border between reinforcement patterns 24, 26 and the flexible printed substrate 21, and reinforcement patterns 24, 26 may pull off from the substrate 21 causing the substrate 21 to break. Pulling off of the reinforcement patterns frequently occurs when there is a big difference between the tensile strength of the reinforcement pattern around the through-holes and the tensile strength of the base material of the printed substrate (particularly with a flexible printed substrate).